I'm not an electrician, just a hobbyist trying to build a small sculpture which incorporates LEDs, but they keep burning out. I am using a 8w/24v Meanwell power supply, a single 20mA Dynaohm resistor, and 3 strings of four 5mm warm white LEDs each, in parallel (see attached diagram). They all lit briefly, but very quickly one string after the next went dark. What am I doing wrong, and how to fix it? THANKS!
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Why are my LEDs burning out?
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What are the specs of the LEDs?
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The OP's "resistor"is a 20mA constant current device.
Surely you shouldn't put three strings of led's in parallel like that?
Any slight difference in the forward voltage drops of the diodes will mean the current is not equally shared between each of the strings. I'd think all the current will go through the diodes with the lowest drop, and the others won't light?
Broughaha - I think you need a Dynaohm in each of the three strings of LED's for your scheme to work.
Can you give details of the led's - recommended running current, forward voltage when running?
Surely you shouldn't put three strings of led's in parallel like that?
Any slight difference in the forward voltage drops of the diodes will mean the current is not equally shared between each of the strings. I'd think all the current will go through the diodes with the lowest drop, and the others won't light?
Broughaha - I think you need a Dynaohm in each of the three strings of LED's for your scheme to work.
Can you give details of the led's - recommended running current, forward voltage when running?
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Thank you all for the very prompt replies. I see that I did not provide enough detail in my original post. I should also mention that I had previously tried using a 12W/15V Meanwell and with that 11 of the 12 bulbs illuminated but the 12th did not. Folks at LEDsupply.com thought I needed more voltage, but then they all burnt out. Below are the product details for the parts I am using:
DynaOhm (mine is the 20mA)
LEDs:
Meanwell Power source (I am using 8w/24v)
DynaOhm (mine is the 20mA)
Product Details
| Output Current: 20mA, 25mA or 30mA | Input Voltage Range: (2.6VDC + Vf LED) to 50VDC |
| Connection: 2 Wire | Output Protection: Open & Short Circuit |
| Maximum Voltage Drop: 30V | Size: 0.25" (Diameter) X 0.5" (L) |
| Control: Pulse & Strobe Capable | Power Dissipation: 0.6W |
LEDs:
Product Details
| Luminous Intensity: 4,700mcd typ. @ 20mA | Power Dissipation: 120mW |
| Max Forward Current: 30mA | Operating Temp: -30 to +85 C |
| Pulse Current: 100mA for <= 10ms, duty <= 1/10 | Soldering Temp: 265 C for 10 secs |
| Forward Voltage: 3.2V typ. 3.5V max @ 20mA | Max Reverse Current: 50uA @ 5V |
| Max Reverse Voltage: 5V | 50° Degree Half Angle |
Meanwell Power source (I am using 8w/24v)
Product Details
| INPUT POWER: 90 ~ 264Vac | CASING: Fully Isolated Plastic Case, IP42 NON PFC |
| OUTPUT VOLTAGE: 5-36VDC | WATTS: 8W up to 35W |
| CONNECTIONS: UL Rated 18-20 AWG Wire | Low Cost, High Reliability |
| WARRANTY: 2-Years | SAFETY: Class 2 Power Unit, Short-Circuit, Over-Load & Over-Voltage |
| Super Small & Compact Size! | APPLICATIONS: LED Lighting & Moving Sign Applications |
Hi Avo I just posted a general reply with the details of what equipment I am using. I'd be very grateful if you take a look at that and then let me know if you think putting a separate dynaohm on each string would be the best approach, or if you think some other arrangement would be more suitable. Thanks!!
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Hi Brouhaha
Thanks for the data. Everything we need is there.
Are your LED's embodied in the sculpture already, or are you still at the experimental stage?
If you still have freedom to play with the arrangement, I would suggest splitting the LED's into two strings of six, each with a Dynaohm in series.
Taking the 'typical' figures, with 3.2V across each lamp, thats 19.2V across six, plus the drop across the Dynaohm of 2.6V = 21.8V so with a 24V supply it should work - BUT if we take 'worst case' of 3.5V across each lamp, that's 21V across all, plus 2.6V = 23.6V, rather close to your power supply output. That's why I think it's best to try it!
You can of course keep to your original scheme of having three strings, but with a smaller number of lamps in each string, I think it's a good idea to have a Dynaohm for each. I was just trying to save you having to buy more Dynaohm's than you need!
If you have a multimeter it would be useful to check you do get these voltages and currents in practice.
Finally I'm not clear why the bulbs should "blow up" unless you connect one the wrong way round, or directly across the supply!
I can see why some may not light when you have parallel strings of a small number of lamps - that's what you need to avoid.
Unless your Dynaohm has blown up while experimenting. It's vital you have a current limiting device like that in each circuit, otherwise you will destroy the bulbs connecting them straight to the psu.
It would be really useful to have a multimeter to check you have 20mA flowing through the Dynaohm(s).
Good luck with the project
Thanks for the data. Everything we need is there.
Are your LED's embodied in the sculpture already, or are you still at the experimental stage?
If you still have freedom to play with the arrangement, I would suggest splitting the LED's into two strings of six, each with a Dynaohm in series.
Taking the 'typical' figures, with 3.2V across each lamp, thats 19.2V across six, plus the drop across the Dynaohm of 2.6V = 21.8V so with a 24V supply it should work - BUT if we take 'worst case' of 3.5V across each lamp, that's 21V across all, plus 2.6V = 23.6V, rather close to your power supply output. That's why I think it's best to try it!
You can of course keep to your original scheme of having three strings, but with a smaller number of lamps in each string, I think it's a good idea to have a Dynaohm for each. I was just trying to save you having to buy more Dynaohm's than you need!
If you have a multimeter it would be useful to check you do get these voltages and currents in practice.
Finally I'm not clear why the bulbs should "blow up" unless you connect one the wrong way round, or directly across the supply!
I can see why some may not light when you have parallel strings of a small number of lamps - that's what you need to avoid.
Unless your Dynaohm has blown up while experimenting. It's vital you have a current limiting device like that in each circuit, otherwise you will destroy the bulbs connecting them straight to the psu.
It would be really useful to have a multimeter to check you have 20mA flowing through the Dynaohm(s).
Good luck with the project
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I'm attaching a photo of the sculpture, taken when it was running on the 15V power supply, and a 30mA dynaohm, and one of the bulbs was not lighting up. I then tried changing to the current power supply, which is 8W/24V, and all the lights illuminated but then one string went dark (they did not blow up or even look different in any way, just stopped lighting. I then rebuilt it, replacing the four dead bulbs, keeping the 8W/24V power supply but changing to a 20mA Dynaohm (at the suggestion of someone at LEDSupply.com) but once again a string of bulbs went dark, followed in short order by the remaining strings. I wish that the bulbs were ok and that I just need to fix something about the power supply and resistor, because replacing the bulbs means a time-consuming rebuilding process, but I am assuming that in fact the bulbs are permanently dysfunctional now and will need to be replaced. So, in answer to your question, I guess I am back to the experimental stage!Hi Brouhaha
Thanks for the data. Everything we need is there.
Are your LED's embodied in the sculpture already, or are you still at the experimental stage?
If you still have freedom to play with the arrangement, I would suggest splitting the LED's into two strings of six, each with a Dynaohm in series.
Taking the 'typical' figures, with 3.2V across each lamp, thats 19.2V across six, plus the drop across the Dynaohm of 2.6V = 21.8V so with a 24V supply it should work - BUT if we take 'worst case' of 3.5V across each lamp, that's 21V across all, plus 2.6V = 23.6V, rather close to your power supply output. That's why I think it's best to try it!
You can of course keep to your original scheme of having three strings, but with a smaller number of lamps in each string, I think it's a good idea to have a Dynaohm for each. I was just trying to save you having to buy more Dynaohm's than you need!
If you have a multimeter it would be useful to check you do get these voltages and currents in practice.
Finally I'm not clear why the bulbs should "blow up" unless you connect one the wrong way round, or directly across the supply!
I can see why some may not light when you have parallel strings of a small number of lamps - that's what you need to avoid.
Unless your Dynaohm has blown up while experimenting. It's vital you have a current limiting device like that in each circuit, otherwise you will destroy the bulbs connecting them straight to the psu.
It would be really useful to have a multimeter to check you have 20mA flowing through the Dynaohm(s).
Good luck with the project
Do you think that there is any chance that the existing bulbs might still be functional and that they would light up again if I fixed some other problem (e.g. replace the Dynaohm or power supply with something different)?
If not, I will try to redesign as you suggest with two strings of six, each with it's own dynaohm (do you think 20maA is the correct size for those?)
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Given your bad luck with this, and the work you've put in, I feel it would be worth getting a cheap multimeter (if you don't already have one), get familiar with it, and every time you connect things up, put the meter in circuit on a suitable milliamp range to check the current is what it should be.
Also you would be able to check the resistance values of the lamps you have, to establish if they still might work before you connect them to power.
My concern is that once things have started to go wrong, you can never be sure what might have been damaged. Are the power supply and dynaohm doing what they should? And if a bulb is out, is it because the wires are shorted together, or the bulb is faulty etc. etc.
Your lamps have an absolute max forward current of 30mA, so I would be inclined to run them at 20mA, unless you need the extra brightness. And if you do, I suspect they won't last as long!
If you can get at the wiring to each lamp, you could check each in turn using a working Dynaohm and supply. Be aware that putting the power on the wrong way round, or supplying excessive current because the Dynaohm is faulty, will instantly destroy the lamp (you won't be able to tell by looking at it).
Your power supplies have protection devices in them, but activated at much higher current/ voltage than your lamps will survive. A safer way of proceeding would be to find a constant current 20mA supply that could give up to 48V, and connect all your lamps in series to that. But I confess I haven't found one yet!
I hope the above makes sense. I would be cautious connecting things up now, as I don't really understand why so many bulbs have failed unless your Dynaohm has too, or the power supply is doing something strange.
Regards.
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Hi Brouhaha
One thing occurred to me - have you connected the Dynaohm the right way round? There's a little "+" on the positive end, and, as supplied, the leads each end are different lengths to also indicate the polarity (don't remember which way round - sorry)
People call it a resistor, but it is a semiconductor device, and only does its job when wired the right way round.
Putting it the wrong way round could destroy the lamps.
And as far as testing/re-using lamps - is the wiring you've done marked with the polarity of the lamps? ie red and black wires, or a tracer mark on one wire, so within the sculpture you know which is anode and which cathode for each lamp?
One thing occurred to me - have you connected the Dynaohm the right way round? There's a little "+" on the positive end, and, as supplied, the leads each end are different lengths to also indicate the polarity (don't remember which way round - sorry)
People call it a resistor, but it is a semiconductor device, and only does its job when wired the right way round.
Putting it the wrong way round could destroy the lamps.
And as far as testing/re-using lamps - is the wiring you've done marked with the polarity of the lamps? ie red and black wires, or a tracer mark on one wire, so within the sculpture you know which is anode and which cathode for each lamp?
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Hi Avo, I have the positive wire coming off of my power supply going to the negative terminal of the Dynaohm--is that correct?
I did color code the wires to the bulbs.
I do have a cheap, old analog multimeter, but I've wondered about it's reliability, so maybe I should get a new one...
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I think this is incorrect. The Dynaohm positive should got to battery positive according to the data sheet.
OH!! That would explain a lot! I am confused however, because I have similarly been wiring the positive power wire to the negative terminal of my LEDs and they do light up when I connect them that way (and not the opposite way). So is there a different convention regarding how the terminals on a resistor are labeled versus those on a bulb?
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As DPG replied earlier, no I'm afraid not!
Brilliant - that means you can check (with multimeter) if any are at least still diodes, rather than open or short circuit!
nothing wrong with old analogue multimeters. mine's 50+ years old and still good enough to use. If yours does resistance measurements, great! (Might need a new battery?)
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You are connecting these the correct way.
The Dynaohm is NOT A RESISTOR
. Sorry, not your fault, it's theirs for marketing it like they do, and everyone calling it a resistor! With an ordinary resistor it doesn't matter which way round it's connected, it does the same thing either way.Whereas the Dynaohm must have some semiconductor devices in it, to alter the voltage across it so that the current through it stays the same.
We should assume the Dynaohm could be damaged, though I think it does have some overload features - I don't know if they save it from reverse connection. If not too costly I would suggest getting another one or two, and putting yours aside for the moment. UPDATE: just looked at the data sheet which says maximum reverse voltage 1V so I suspect it's broke!
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I'd take Avo's advice. At the minute you have a Dynaohm of unknown condition, and LEDs which may have been damaged. Get a new Dynaohm, and some known good LEDs. Connect them up correctly, maybe in a string of 4, and test them.
Use your multimeter to measure LED current if it will do DC amps. Monitor it at switch on and be ready to switch off if the current is too high.
Use your multimeter to measure LED current if it will do DC amps. Monitor it at switch on and be ready to switch off if the current is too high.
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Thank you Avo and DPG for your very helpful replies! It will probably be a few weeks before I can get the replacement Dynaohm and have an opportunity to try testing/rebuilding, but I will come back here either with a success story--or more questions! Thanks so much!
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DPG gives good suggestions in his last post.
Just to reiterate, don't wire it up as your original drawing back in post #1. You need 20mA through each string of lamps, so however many you end up with in series, four or six, have a Dynaohm for each string. Perhaps they'll give you a loyalty discount
Best wishes for getting it all working!
ok, I got new 20mA dynaohms, new LEDs and started over; I made three parallel strings of 4 LEDs and tested each string using a couple of 9v batteries (and a Dynaohm, although it's possible that I might have left out the Dynaohm at some point while testing...) maybe that caused my subsequent problem?). I then wired everything up and tried powering it with my Meanwell power source and found that one of the bulbs in one of the strings is not lighting up--but the other three bulbs in that string are lighting up. I don't even understand how that is possible, I thought if one bulb was bad the whole string would go dark. Any idea how that could happen? And any ideas why the strings were working before I wired up to power source but not afterward?DPG gives good suggestions in his last post.
Just to reiterate, don't wire it up as your original drawing back in post #1. You need 20mA through each string of lamps, so however many you end up with in series, four or six, have a Dynaohm for each string. Perhaps they'll give you a loyalty discount
Best wishes for getting it all working!
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Sorry you still have problems.
regarding the LED not lighting up, if a diode fails 'short circuit', the others will carry on working, and the Dynaohm will adjust the current to still be correct for the others.
It could be the diode legs, or the wires to that diode, are shorted together.
It could be that you've inadvertently wired it the wrong way round, and as a result its gone short circuit (but in that case it would never have lit up when you first tried it).
It could be that the diode has just failed short circuit.
Suspect you need to replace that one diode unfortunately, if that is practicable.
ah--I hadn't realized that current could still pass through a dead bulb. I will replace that one (again!) and hopefully eventually get back here to report success!
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Yes unlike a traditional filament lamp which fails open-circuit like a fuse, an LED typically fails short-circuit especially if due to overcurrent, as the diode semiconducting junction breaks down and becomes fully conductive. Unless it has been blasted with such a high current that the bond wires have ruptured.
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